Tài liệu mạng máy tính nâng cao application layer protocol 2

Application Layer
Protocols 2

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Today's objectives


More on FSMs





aggregate state machine
checking properties

More application-layer protocols





remote login
quick introduction to security
ssh: secure shell
peer-to-peer networks

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Application layer
Host

Router

Host

Application

Application

Transport

Transport

Network

Link/Physical
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Network

L/P

L/P

Network

Link/Physical
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Intersection server FSM #1
RED

rcv_msg (cl, RED)
------------------------------send_msg ([NSEW \ cl],
ALL_RED)

rcv_msg(cl, NEED_GREEN)
--------------------------------send_msg (cl, OK_GREEN)

ONE_GREEN
rcv_msg(cl, NEED_GREEN)
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Intersection client FSM #1
from_detector (car waiting)
--------------------------------------send_msg(server, NEED_GREEN)
RED

CAR_WAITING

timer expires
------------------------------set_light (red)
send_msg (server, RED)

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GREEN

rcv_msg(OK_GREEN)
--------------------------------set_timer (20sec)
set_light (set green)

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Joint FSM
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Joint FSM:





An FSM that captures the evolution of the system as a whole
Each state S of the joint FSM corresponds to the set of states
each component FSM 1,...,n is in:

 S=(S1,S2,...,Sn)
Every transition (S1,S2,...,Sn) -> (S1',S2',...,Sn') corresponds
to one or several transitions in component FSMs
 If several transitions, they have to be concurrent

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Joint FSM for intersection protocol #1


State = (server, state_client1, state_client2)
(OG,
R,CW)

(R,
R,CW)

(R,
CW,R)

(OG,
CW,R)

(R,
R,R)
(OG,

R,G)

(OG,
CW,CW)

(OG,
CW,G)

(OG,
R,R)

(OG,
R,R)

(R,
CW,CW)

(OG,
CW,R)
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(R,
CW,R)

(R,
CW,CW)

(R,
R,CW)

(OG,
G,R)

(OG,
CW,CW)

(OG,
R,CW)
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(OG,
G,CW)
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Joint FSM for intersection protocol #1

(OG,
R,CW)

(R,
R,CW)

(R,
CW,R)

(OG,
CW,R)

(R,
R,R)

(OG,
R,G)

(OG,
CW,CW)

(OG,
CW,G)

(OG,
R,R)

(OG,
R,R)

(R,
CW,CW)

(OG,
CW,R)
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(R,
CW,R)

(R,
CW,CW)

(R,
R,CW)

(OG,
G,R)

(OG,
CW,CW)

(OG,
R,CW)
/>
(OG,
G,CW)
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Joint FSM for intersection protocol #1


Classes of states


We can leave class 1 to either class 2 or 3 and never come
back!
Class 1

Class 2

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Class 3

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Intersection client FSM #2, no blocking
from_detector (car waiting)
--------------------------------------send_msg(server, NEED_GREEN)
RED
CAR_WAITING

timer expires
------------------------------set_light (red)
send_msg (server, RED)

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GREEN

rcv_msg (ALL_RED)
--------------------------------------send_msg(server, NEED_GREEN)

rcv_msg(OK_GREEN)
--------------------------------set_timer (20sec)
set_light (set green)

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Joint FSM for intersection protocol #2


State = (server, state_client1, state_client2)
(OG,
R,CW)

(R,
R,CW)

(R,
CW,R)

(OG,
CW,R)

(R,
R,R)
(OG,
R,G)

(OG,
R,R)

(OG,
CW,CW)

(OG,
CW,G)

srv: rcv(cl2, NEED_GREEN)
-------------------------------------srv: send(cl2, OK_GREEN)

(R,
CW,CW)

(OG,
CW,R)
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(R,
CW,R)

(OG,
R,R)

(R,

cl2: rcv(ALL_RED)
CW,CW)
--------------------------------------cl2: send(srv,NEED_GREEN)

(R,
R,CW)

(OG,
R,CW)
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(OG,
G,R)

(OG,
CW,CW)

(OG,
G,CW)
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Joint FSM for intersection protocol #2


Single class of states


We can go from anywhere to anywhere, given the right “input”

Class 1

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Another example of FSM-based analysis
condition C
----------------action D
condition A
----------------action B

X

condition C
----------------action D

Y
condition A
----------------action B

is equivalent to:



condition A
----------------action B

Z

condition C
----------------action D

All that matters is input->output



Can we get rid of internal states?
Automatic simplification
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FSM: summary


Individual FSM:




Describes individual protocol entity

Joint FSM:








Can be generated automatically
Describes communicating set of entities
Protocol verification: check properties such as “no absorbing
classes of states”; if such a class exists, what is sequence of
events that leads there, etc.
Advantage:
 Allows mathematical proof of these properties

 Software tools, code generators
Disadvantage:
 Computational complexity quickly becomes large as FSMs
become more complex -> number of joint states explodes
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Remote session: telnet, rlogin, ssh



Work on a remote system
Applications



telnet
R-commands of Unix






rlogin, rsh, rcp

ssh

Principles




send characters typed on the keyboard to the remote
shell
receive characters from the remote shell and display

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rlogin, telnet, ssh

t eln et
clien t

t er m in al
dr iver

user

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t eln et
ser ver

TCP/ IP

TCP/ IP

pseudo
t er m in al
dr iver

sh ell

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rlogin


Connection between UNIX systems




port 513

Authentication



password (send in clear)

.rhosts file





host name
user name

Typed character


echo sent by the remote system

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telnet


Connection between any system


NVT (Network Virtual Terminal)



common denominator between different systems
NVT ASCII
–
–




Port 23
Options




7 b i t s , en d of l i n e: CR, LF
al s o u s ed b y FTP, SMTP, f i n ger , w h oi s , HTTP

character mode, line mode

Escape - interpreted by the client


Control-]

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SSH: what is network security?
Confidentiality: only sender, intended receiver
should “understand” message contents

sender encrypts message

receiver decrypts message
Authentication: sender, receiver want to confirm
identity of each other
Message Integrity: sender, receiver want to
ensure message not altered (in transit, or
afterwards) without detection
Access and Availability: services must be
accessible and available to users
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Friends and enemies: Alice, Bob, Trudy




Well-known in network security world
Bob, Alice want to communicate “securely”
Trudy (intruder) may intercept, delete, add
messages
Bob

Alice

ch an n el dat a, con t r ol
m es s ages

dat a

secur e
sen der

secur e
r eceiver

dat a

Tr udy
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Who might Bob, Alice be?

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

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… well, real-life Bobs and Alices!
Web browser/server for electronic transactions
(e.g., on-line purchases)
on-line banking client/server
DNS servers
routers exchanging routing table updates
other examples?

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There are bad guys (and girls) out there!
Q: What can a “bad guy” do?
A: a lot!
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eavesdrop: intercept messages
actively insert messages into connection
impersonation: can fake (spoof) source address in

packet (or any field in packet)
hijacking: “take over” ongoing connection by
removing sender or receiver, inserting himself in place
denial of service: prevent service from being used by
others (e.g., by overloading resources)

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The language of cryptography
KA

plain t ext

Alice’s
encryption
key

en cr ypt ion
algor it h m

ciph er t ext

KB

Bob’s
decryption

key

decr ypt ion plain t ext
algor it h m

symmetric key crypto: sender, receiver keys
identical
public-key crypto: encryption key public, decryption
key secret (private)
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Symmetric key cryptography
substitution cipher: substituting one thing for
another


monoalphabetic cipher: substitute one letter for another
plaintext:

abcdefghijklmnopqrstuvwxyz

ciphertext:

mnbvcxzasdfghjklpoiuytrewq

E.g.:

Plaintext: bob. i love you. alice
ciphertext: nkn. s gktc wky. mgsbc

Q: How hard to break this simple cipher?:

brute force (how hard?)
other?
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Symmetric key cryptography
K
A- B
plain t ext en cr ypt ion ciph er t ext
m essage, m algor it h m
KA-B(m)

K
A- B
decr ypt ion plain t ext
algor it h m
m = KA-B(KA-B(m))

symmetric key crypto: Bob and Alice share know
same (symmetric) key: KA-B




e.g., key is knowing substitution pattern in mono
alphabetic substitution cipher
Q: how do Bob and Alice agree on key value?
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